Some time before the outbreak of World War Two serious consideration was given to the fact that aircraft electrical wiring might create serious bottlenecks in production, unless a system could be evolved to make it possible to pre-assemble a simple, complete wiring-harness in widely-dispersed factories. It was also felt that simplicity, lightness and ease of maintenance in service under war conditions would be factors of primary importance.
The pre-war practice of working to a wiring diagram and wiring each aircraft separately was both undesirable and impracticable for wartime methods of manufacture under dispersal conditions. With these and a number of other major considerations in mind, a wiring system of the ‘open wiring’ type was evolved and tested by Vickers-Armstrongs (Supermarine) Ltd., with the cooperation of Joseph Lucas Ltd. The pins required for this type of work of special moulded-plastic material for lightness and non-conducting, were developed by Oddie, Bradbury and Cull Ltd.
The tests proved to be so successful that it was decided to adopt this form of wiring for production, and its advantages have proved themselves in service over a long period, under extreme conditions. This system was based upon the use of single-core cables, bunched in looms, over which were passed short lengths of poly-vinyl-chloride sleeves. A hole was punched in the PVC sleeve, to take the special pins which, in turn, were pushed through the cables. The harness, or loom, was attached to the airframe by the Oddie pins which engaged in special clips riveted to the airframe structure. The ends of the separate cables engaged in their appropriate connectors.
A resilient washer was fitted under the head of the pin on the original installation, so that should it be found necessary to introduce additional cables on later modifications, this washer could be removed and the original pin used on the increased thickness of the loom.
Workers assembling looms on jig-boards which were inclined at an angle for greater convienience in wiring and at a height to eliminate unnecessary effort.
For the first stage the cables were cut off to the correct lengths. This operation was done on simple jigs, the cable being fed automatically, and guillotined. The cable lengths were then stripped on a special electrically heated device, which ensured that the stripping was carried out without damage to the conductors. Next, the cable lengths were fitted with their appropriate connectors, terminals or eyelets. There were a number of ingenious but simple machines which perform these operations.
Identification of the cables was the next step and was carried out by lettered and numbered adhesive tape, specially developed for this wiring system, and produced by Cable Assemblies Ltd., a subsidiary of Herts Pharmaceuticals, Ltd. Completed cable lengths were then grouped in racks in their appropriate lengths in readiness for forming the looms.
The PVC sleeves, approximately one to two inches in length, and of various diameters, were usually manufactured by Tenaplas Ltd., They were cut to length in a single process on a simple press from continuous tube, the hole for the pins punched, and the lines indicating the locked position of the pins marked.
A close-up of one of the completed looms on the assembly jig board.
